OpenEuphoria

Re: Is subscripting a slice useful

• Posted by Pete Lomax <petelomax at blueyonder.co.uk> Oct 12, 2004
• 582 views

On 11 Oct 2004 11:56:47 +0200, Christian Cuvier
<Christian.CUVIER at agriculture.gouv.fr> wrote:

>You are using a typo from the original message to make your point 
Would that be the same typo mercilessly repeated no less than NINE
times in a subsequent post? )
>about your personal dislike of this construct.
I'm entitled to an opinion, last time I checked.
>
>> There is *no* way that asdf[1..2] should return anything other than a
>> sequence of length 2,
>
>True
>
>> and hence a nested subscript of that could not
>> possibly yield a sequence of length 4. 
>
>False, if the intermediate sequence is made of sequences itself.
I don't understand what you mean. Please give me an example.
>
>> This whole idea (which has been extensively discussed before) is so
>> open to misinterpretation it should definitely NOT be "standard".
>> Every application needs something different.
>
>Really? Most of them need vertical slicing anyway, so it is desirable for it 
>to become a standard. Could you give some examples of different schemes that 
>might be concurrently needed?
The complexities this introduces worry me, and I fear it is not
flexible enough to justify it. Suppose that
c =
{{{2,4,6,20}, {8,10,12,22}, {14,16,18,24}},
 {{3,6,9,30}, {12,15,18,33}, {21,24,27,36}},
 {{24,25,26,33}, {27,28,29,34}, {30,31,32,35}}}

How do you get {8,14,12,21}?

For that matter, and to spoil your fun, what is c[1..2][2..3][1..1]?

Can you explain to me why that should _not_ be
{{{8,10,12,22}}, {{12,15,18,33}}} ?

Also, do you believe that eg

	c[2..3][2][3..4]+=1

should be supported?
If so, what should the result be?
>
>[snipped remainder]
>
>Consider a sequence with at least two levels, and look at how you access it.
>Each index specification (like [3]) just selects one element. Each slice 
>specification (like [1..3]) selects a *range* of elements.
>Vertical slicing is not ambiguous if you consider that [3] and [3..3] are 
>unambiguously referring to different objects. Slices operate on matrices 
>(well, tensors in dimensions >2), and slices selectt individual elements as 
>probably expected.
>
>To come back to the original example above:
>
>-asdf[1..2][1] would print {97,97}: select element #1 in all subsequences of 
>asdf[1..2].
>-asdf[1..2][3..4] would print {{100,102},{100,102}}: slice [3..4] of all 
>elements of asdf[1..2].
>-asdf[1][1..2] would print {97,115}, as usual.
Yes, that's fine.
>
>Could you elaborate an example where this extended slicing, compatible with 
>the current EU syntax, could lead to two different interpretations?
Someone once posted that a[x..y][z] would just return a[x+z-1], and
there is certainly some logical reasoning to that, though of course it
would be utterly pointless to make a change which works that way.

Maybe something like:
c =
{{{1,1}, {1,1}, {1,1}}, {{1,1}, {1,1}, {1,1}}, {{1,1}, {1,1}, {1,1}}}

 c[2..3][2..3]+={2,3}

could give

{{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,3}, {4,4}}, {{1,1}, {3,3}, {4,4}}}

or

{{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,3}, {3,3}}, {{1,1}, {4,4}, {4,4}}}

or

{{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,4}, {3,4}}, {{1,1}, {3,4}, {3,4}}}

To be honest I have no idea which would be correct.

Regards,
Pete